Growth of single crystals



W. W. PIPER Pfg/ Filed July 51, 1964 GROWTH oF SINGLE CRYSTALS March 29,1966 In Verv'tor'.- Wf//fd W. Piper-3 H Attorney United States Patent O3,243,267 GROWTH F SINGLE CRYSTALS William W. Piper, Scotia, NX.,assigner to General Electric Company, a corporation of New York FiledJuly 31, 1964, Ser. No. 386,505 Claims. (Cl. 23e-301) The presentinvention relates to an improved method for the production of singlecrystals of semiconductive materials which may not be grown by simpleseed crystal withdrawal from a mol-ten bath. Such materials are, forexample, useful as luminescent phosphors and as photoconductingmaterials. This application is a continuationin-part of my copendingapplication Serial No. 19,295, tiled April l, 1960, and assigned to theassignee of the present invention.

In the luminescence arts, it is well known that luminescent materialssuch as zinc sullide, cadmium sulfide and other members of thezinc-cadmium sulffo-selenide family, as well as zinc oxide, may be madeto produce visible light when suitably activate-d with acceptor typeactivators and donor type co-activators and are subjected -to a suitablestimulus as, for example cathode rays, ultraviolet light or theexcitation of an electric eld. In most applications of these materials'for the production of light as, for example, in the viewing face ofcathode ray tubes, the luminescent phosphor is present in the form of aplurality of small or micro-crystals lwhich may be in powder formpressed, liquid settled or sprayed into a thin layer `or suspended in asuitable binder or dielectric medium.

In my U.S. Patent No. 2,841,730', it has been shown that increasedluminescent etliciency may be obtained if the phosphor is in the form ofa large single crystal or is composed of a plurality of members cut froma monocrystalline ingot of the phosphor material. Heretofore, however,it has been impossible to grow true single crystals of these compoundsin large sizes. It has also been impossible to grow large crystals or"these substances for use in photoconducting devices. The techniqueswhich are utilized to grow monocrystalline ingots ofi monatomicsemiconductive materials, such as germanium and silicon, `are notapplicable for the growth of single crystalline ingots of thesecompounds. The only methods previously utilized in growing true `singlecrystals of these compounds has been by sublimation crystal growth inwhich the crystals are grown in the form of small needles harving amaximum length and width dimension of the order of a millimeter. It isdillicult to construct large luminescent or photoconducting `devices orelectro-optical transducers from such small crystals.

Accordingly, it is an object of the present invention to provide animproved method for the production of single crystals of compoundsemiconductive materials.

Another object of the `present invention is to provide a method for theproduction of single crystalline ingots of luminescent phosphor andlphotoconducting compounds sulliciently large so yas to facilitate thefabrication of monocrystalline luminescent, :photosensitive andelectro-optical devices.

In accordance with one embodiment of the present invention, largemonocrystalline ing-ots of compound semiconductive material-s are grownfrom the vapor phase in a closed tube having a configuration at one endthereof which kfacilitates single crystal nucleation and which isprogressively moved through a stationary temperature proiile tofacilitate progressive grow-th of a large single crystalline ingot bynucleation at an interface which is maintained at a constant temperaturemost suitable tor crystal nucleation.

In accord with another feature of the invention, the tube Within whichthe crystal is grown is not evacuated, but rather, is filled by an inertgas at approximately ICC one atmosphere admixed lwith vapors of thematerial constituting the charge from which the crystal is grown. Theopen end of the reaction tube, upon equalization of the pressures withinland without the growing tube, is sealed by condensation of these vaporsduring the initial stage of operation thereby preventing undue straindue to unequal pressures across the hot, soft tube wall.

The novel features believed characteristic of the present invention areset forth in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood byreference to the following description, taken in connection with theappended drawing in which:

"FIGURE l represents a schematic illustration of an apparatus suitablefor the Ipractice of the present invention,

`FIGURE Z illustrates an enlarged vertical crosssectional view of .thereaction tube =of .the apparatus of FIGURE 1, and

FIGURE 3 is a graph illustrating a typical temperature lprotilemaintained along the tube of FIGURE 1 in the practice of the invention.

yIn FIGURE l of the drawing, a suitable apparatus within which thepresent invention may be practiced comprises a first high-temperaturereaction tube 1 which contains a seco-nd high-temperature reaction tube`2 having a main cylindrical region 3 and a conical end region 4terminated in a point 5. Means for closing the open e-nd of tube 2 isprovided in the form of a rporous plug 6. The central region of reactiontube 1 is surrounded by a plurality of resistance yheating coils 7, I8,9 and 1li, -to which heating currents may be applied at the respectiveterminals thereof. The open end of tube 1 is closed with a tapered,ground-glass stopper, suitable for makin-g a vacuum-tight seal, andwhich has passing therethrough a pair of tubes 12 and 13. Reaction tube1 should be capable of withstanding very high temperatures of up toapproximately 1600 C. and may be conveniently constructed of mullite,which is aluminum silicate (Al6Si2'O13). Interior reaction tube 2 shouldbe constructed of a material which need not be yas refractory as thematerial from which tube 1 is constructed and preferably is of amaterial, such as quartz, which is not wet by condensing vapors of thecompound semiccnductive material, single crystals of which are to begrown.

Plug `6 may be in the form of a variety of structures andcontigui-ations which are not particularly critical but which mustperform the function of Iformi-ng a passage to gas and vapors of thematerial being grown within tube 2 so as to allow the pressures `acrossthe tube to equalize and to present a surface for condensation of thegenerated fvapors, the condens-ation or" which eventually causes thatend of reaction tube 2 to be sealed. In practice, plug 6 may constitutea tightly packed mass of quartz wool. Alternatively, it may comprise acompressed mass of quartz plug having an outside dimension only slightlysmaller than the inside dimension of tube 2, so that condensation ofvapors of the charge takes place -upon the interior surface of tube 2and the exterior cylindrical surface of the plug. For Iconvenience indenominating the many and varied structures which plug 6 may take, theplug may be referred to herein as closure means for allowing chargevapors and carrier gases to escape from the tube and for sealing thetube by an accumulation of condensed vapors of the charge.

This closure means is of particular importance in the present inventionbecause it allows equalization of the pressures within and without thetube 2. The optimum material for this tube is quartz because it isreadily available, workable and inert. However, at the temperatures atwhich the present invention is performed, quartz becomes quite soft and,under unequal pressures, may deanziane? form or even explode. Theclosure means of the present invention prevents such an event 'byallowing communication, and therefore equalization, of the pressuresacross the tube 2.

In other words, if tube 2 were sealed, the increased pressure of theinert gas due to heating and the added pressure of evolved charge vaporswould be suiiicient to deform or break'the quartz tube. The closuremeans avoids this problem by allowing the inert gas to pass and achievepressure equilibrium during heating and by allowing the inert gasdisplaced by evolved vapors to escape and maintain pressure equilibriumbefore the tube is `sealed ott by the condens-ation of vapors on theclosure means.

The arrangement `of reaction tubes 1 and 2 and heater coils 7, 8, 9 and10 are such as to allow for the establishment of a suitable temperatureprole, described in greater detail hereinafter to permit motion of tube2 through the stationary profile. To this end, several variations may beutilized. Thus, for example, the coils may be wound upon tube 1 and tube2 may be slidable therein, either on rollers or by friction at the,relatively small line of contact between the two different diametertubes. This arrangement makes it possible to locate the heating coilsvery close to the growing crystals and facilitates optimum control ofthe temperature profile. Alternatively,

the coils may be wound upon a third cylindrical core (not shown) andtubes 1 and 2 may be slidably moved as a unit therein, as above. Thisarrangement has the advantage of moving the two tubes, one of which isseated within the other, together. l

In FIGURE 2 of the drawing, the portion of the apparatus in which themonocrystalline ingot 14 is grown is shown in enlarged detail. In FIGURE2, like numerals have been Vutilized `to indicate like elements alsoillustrated in FIGURE 1. A charge 15 of the material from which themonocrystalline ingot isto be grown is placed immediately adjacent toplug 6. While this charge may be merely a powderedmass of the materialfrom which the ingot is to be grown, it has been found preferable thatcharge 15 be a sintered mass of powdered material which has been heatedto a sufficiently high temperature to cause the particles to agglomeratetogether, giving'a rigid form and a highderisity thereto. When thematerial utilized for the charge iszinc sulfide, this may beaccomplished by heating powdered luminescent grade zinc suliide to atemperature of approximately 1000 C. in H25 gas at 1 atmosphere pressurevfor approximately 1/2 hour.l The resultant body has a density ofapproximately 3.2 to 3.5 and contains approximately to 20% voids, theremainder being solidcrystalline zinc sulde. Corresponding densities forcadmium sulfide are approximately 3.8 to 4.3 and approximately 4.8 'to5.3 for cadmium selenide, for example. These densities, of approximately3 to 5 for a sintered charge, compare with a representative density forpowdered charges of from l to 1.5. The increased density of the chargeis advantageous in that a larger amount of charge may be incorporatedinto the tube in a smaller space thus facilitating the task of alwayskeeping the portion of the charge being sublimed at a sufficiently hightemperature, while still keeping the temperature at which plug 6 islocated at a necessary low value. Charge 15 is composed of the samematerial of which monocrystalline ingot 14 to 'be grown. The presentinvention may be practiced with semiconductive materials exhibitingluminescent and/or photoc'onductive properties and composed of compoundsof elements of groups IIb and VIa of the periodic table. Such materialsinclude simple and complex Compounds of the zinc-cadmium sulfo-selenidefamily including Zinc sulfide, cadmium sulfide, zinc selenide, cadmiumselenide, zinc-cadmium suliide, zinc-cadmium selenide, zincsulfo-selenide,vcadmium sulfo-selenide, zinc-cadmium sulfo-selenide aswell as zinc oxide and cadmium oxide. The main substantial differenceinpracticing the invention withy these diifer'ent materials is that thetemperature profile established within the reaction tube must beadjusted upwardly or downwardly to compensate or adjust for thediffering vapor pressures at a given temperature of the charge materialutilized. This is because the mechanism by which the process ofthe'invention operates relies upon the establishment of a partialpressure of the charge material within the reaction tube 2 bysublimation. While the exact value of this partial pressure is not knownwith certainty, it is believed to be approximately 0.1 atmosphere. Theregion at which a monocrystalline ingot is` grown is maintained at atemperature only slightly below the temperature of the charge butsufficiently low enough to cause the above partial pressure to result ina suicient degree of supersaturation at point 5 to `cause nucleation.Thus it may be seen, that the specific temperature sulicient to causethe desired partial pressure within reaction tube 2 varies with thematerial used. It is to be noted, however, that when a complex salt 4isutilized as a charge, it is not merely a mixture of the constituentsalts, but

v rather, the complex salt is present in crystalline form so as to be asolid solution of one of the constituents in another.

A temperature prole suitable for the growth of single crystals of theluminescent phosphor compounds utilized is established within thereaction tube 1 by a proper adjustment of the configuration of, and thecurrents passed through, heaters 7-10. Thus, for example, it has beenfound convenient, in the illustrated contiguration, when the outsidediameter of reaction tube 1 is 18 millimeters and the length of theinterior reaction tube 1 from tip 5 of conical region 4 to the beginningof the nearmost edge of plug 6 is 11 centimeters, that coils 7 and 1t)may be 21/2 in length and have 8 turns per inch of 0.030 platmum-10%rhodium wire. Similarly, under the same circumstances, coils 8 and 9 mayconveniently be 3% in length and have 12 turns ,per inch of the samewire.

The voltages applied to coils 7, 8, 9 and 10 are chosen to provide atypical temperature profile such as is illustrated in FIGURE 3 of thedrawing, comparing FIG- URES 2 and 3 together. The temperature profilein FIG- URE 3 corresponds to the corresponding longitudinal positionalong tube 2 of FIGURE A2. This gradient is chosen to have a relativelygentle slope from beyond the innermost portion of tube k1 at the endremoved from plug 11 that rises gradually, has a somewhat steeper slopebeginning at point A, levels off at point B, begins to decrease at pointC, and at point D, has fallen to a temperature value somewhat below thetemperature at point A. The exact position of points A, B, C and Drelative to tube 2 are chosen so that point N, a point midway betweenpoints A and B corresponds to the tip 5 of tapered portion 4 of tube 2just before crystal growth is initiated. Additionally, the hightemperature region between points B and C extends inwardly into the tubeand includes the forward edge of charge 15. Point E, at a temperaturesubstantially below the condensation point of the material constitutingcharge 15, corresponds substantially to the portion of the tube at thebeginning of plug 6. Point N, between points A and B, serves as thenucleation point for the monocrystalline ingot being grown andcorresponds in temperature to the temperature at which vaporizedmaterial of the charge begins to condense. The temperature of the tubein the region including points D and E and extending to the end of thetube decreases rapidly so as t`o favor rapid condensation of the chargematerial in or around plug 6.

As examples of temperatures which may be utilized in performing theprocess of the invention utilizing the described apparatus and typicalmembers of the zinccadmium sulo-selenide family of materials, lfor zincsuled point A is approximately 1520 C. For cadmium sulr'ide it isapproximately 1220 C., and for cadmium selenide it is approximately 1l20C. Point B for zinc suliide is approximately 1550 C., while for cadmiumsulfide it is approximately 1250 C. and for cadmium selenide it isapproximately 1150 C. Point D is approximately 1400 C. for zinc sulfide,1100 C. for cadmium sulde and a 1000 C. for cadmium selenide.

In practicing the invention, one difficulty which must be overcome isthat of cooling the growing ingot sufficiently so tha-t the heat of thefusion is removed therefrom so as to maintain the interface temperatureat a value which is optimum for nucleation. One method of accomplishingthis is illustra-ted by the addition, to tube 2 of glass rod 16 whichextends outward of the field of infiuence of heater coils 9 and 10 andconstitutes a heat sink. Other means for creating a heat sink nearcrystal 14 for removing heat therefrom may be utilized. Thus, forexample, a blast of cool air may be directed upon the adjacent portionof tube 1. Alternatively a cooling coil may encircle tube 1 in thisregion. Other well known cooling means may be provided.

In performing the growth of large single crystals of compoundsemiconductive materials of the zinc-cadmium sulfo-selenide family andoff theoxides of zinc and cadmium in accord with the present invention,a charge is made by sintering, as described hereinbefore. The charge isplaced within tube 2 and plug 6 is inserted at the end thereof. Tube 2is then inserted in tube l. Plug 11 is inserted in the end of tube 1 andthe atmosphere therein is evacuated to a pressure which may convenientlybe less than 1 micron of mercury while the tubes are heated to atemperature of approximately 500 C. to remove therefrom any atmosphericconstituents which may react with, or contaminate the vapors of, thecharge. This evacuation may conveniently be conducted through tube 12 bya vacuum pump, not shown. When a suitable value of low pressure has beenobtained within the charnber, a suitable non-reactive gas such as argonor any other of the noble gases is admitted to tube 2 at a pressurewhich may conveniently be approximately 1 atmosphere, through tube 13and is allowed to escape therefrom through tube 12. Alternatively, theevacuation step may be dispensed with, and the atmosphere within tube 1may merely be purged by fiushing, while heating, a suitable -gas such asargon into tube 13 and allowing the gas to escape through tube 12, for asuitable period of time, which may for example be 5 hours, to removenon-reactive atmospheric contaminants.

After a suita-bly pure atmosphere of a non-reactive gas has beenestablished within tube 1, suitable voltages are applied to coils 7, 8,9 and 10 to establish a temperature profile such as that illustrated inFlGURE 3 and described hereinbefore. Since the temperature within tube 1at which the inner edge of the sintered charge 15 is located is higherthan the temperature at which the material of the charge sublimes,vapors of the charge material rapidly fill the interior of tube 1 to asuitable partial pressure of, for example, approximately 0.1 atmosphere.TheV temperature at the vicinity of charge `15 is, however, maintainedat a temperature only slightly in excess of that necessary to causesublimation so that the vapor pressure of the charge material does notbecome too high. As -the interior of tube 1 is filled with vapors ofcharge 15 the vapors begin to leak out through or past plug 6. Since thetemperature in the vicinity of plug 6 falls rapidly as the end of thetube is approached, the vapors escaping through or past plug 6 rapidlybegin to condense thereupon. After a relatively short time, these vaporshave deposited in or around plug 6 to the extent that the end of thetube is sealed, thus preventing the escape therefrom of any furthervapors. By that time impurities, air and other undesirable vapors havebeen flushed from the system.

As previously noted, the use, in accord with the invention, of asealable plug 6 or other closure means is of importance. All materialsknown to be suitable for use as reaction tubes become soft `at thetemperatures required to sublime IIb-Vla materials and may deform if apressure differential exists between the interior of tube 1 and tube 2.Accordingly, the novel closure means allow gas expansion and vaporevolution of the charge to force out sufficient inert gas to equalizethe pressures across tube 2 and thereafter the temperature gradientestablished is maintained so that the pressure differential between theinside and outside of reaction tube 2 does not exceed a small value of,for example, 50 millimeters of mercury.

It has been found in performing this invention that sublimation of thesintered powder occurs back into the cool region adjacent the plugthereby forming a solid, high density, polycrystalline boule. Thisresults in a first purification of the material. When the leading edgeof the solid charge enters the sublimation zone, vapors evolve therefromand a second purification occurs. If, upon completion of crystal growth,a final portion of the boule adjacent the plug is not sublimed, theresultant crystal is found to be much cleaner than is the case if theentire boule is vaporized.

The temperature of tip 5 is so selected that the partial pressure ofcharge vapors in the reaction tube 1 creates a supersaturated conditionat that point and nucleation begins at the pointed tip 5 of tapered end4 to tube 2. One single crystal does not necessarily nucleate to theexclusion of all others. Several crystals may actually nucleatesimultaneously. Should such a condition exist, however, one or twocrystals crowd out the remainder and these continue to grow and fill theentire diam-eter of the tube. If several crystals grow, a bi-crystal ortri-crystal results, but the ingot is so large that two more crystalsmay readily be cut apart. Once nucleation has been commenced and crystal14 begins to grow, tube 2 is moved longitudinally within tube 1 so thatthe growing crystal interface is always located at a point correspondingto point N intermediate to points A and B on the curve of FGURE 3 of thedrawing, which point corresponds to the most favorable nucleationtemperature for the material of which the single crystal is being grownand the degree of supersaturation.

As the tube 2 is moved through the temperature profile, plug 6 entersinto a region having a temperature higher than initially. This is,however, not important since the tube has already been sealed and theincreased temperature present at plug 6 is controlled so as to beinsufficient to allow the tube to be opened by sublimation. It isnecessary, however, should an extremely long crystal be grown, that asplug 6 progresses inwardly, the temperature profile be adjusted so thatthe condensed material in and around plug 6 does not sublime, thusdestroying the seal of tube 2.

The rate at which tube 2 is moved corresponds to the rate of crystalgrowth. Although this rate is not critical within an order of magnitude,it has been found that it is relatively slow. For example, if tube 1 hasa crosssectional diameter of 13 milli-meters and the material beinggrown is zinc sulfide or cadmium sulfide, it has been found that therate of travel, and consequently the rate of crystal growth, should becontrolled to be Within the ranges of from 0.1 to 2 millimeters perhour. Under these conditions and with these materials, it has been foundthat ideal conditions obtain and maximum size single crystals are grownwhen the rate of crystal growth is approximately 0.5 millimeter perhour.

Crystal growth is continued as described above until substantially allof the charge is exhausted. After the growth of the crystal has ceased,the coils are disconnected from the voltage supply and the reactionapparatus allowed to cool. Since the compounds of the zinc-cadmiu-msulfo-selenide family as well as Zinc and cadmium oxides do not readilywet a quartz tube and since the coefficients of thermal expansion ofthese materials are greater than that of quartz, when the tube cools thesingle crystal shrinks away from the Walls of tube 2 and may readily beremoved therefrom.

Utilizing the method of crystal growth of the present invention, singlecrystals of zinc or cadmium oxides, as well as zinc sulfide, zincselenide, cadmium sulfide, cadmium selenide, and complex saltstherebetween having diameters up to 13 millimeters and lengths as highas 20 millimeters have been grown. These crystals have been found tohave excellent crystal perfection, high purity, and, when suitablyactivated, may be made photo-sensitive or luminescent when energized bycathode' rays, ultraviolet light or an electric field. Such activationmay readily be accomplished by cutting a single crystalline wafer fromthe grown ingot and diffusing suitable luminescence activators such ascopper, silver, gold, and manganese, and suitable luminescenceco-activators such as chlorine, bromine, iodine, aluminum, galliu-m, andindium substantially uniformly throughout the wafer. In one means foraccomplishing this, the activator and co-activator may be vacuumevaporated upon one surface of the crystal and the crystal heated for atemperature and time suitable to diffuse the activator and co-activatoruniformly throughout the crystal or throughout a wafer cut from thecrystal. Additionally, if the charge is an activated luminescentphosphor, it has been found possible to sublime and condense, inmonocrystalline form, the entire activated phosphor in the sameproportion of host and activator.

In producing monocrystalline ingots in accord with the presentinvention, it has been found essential that the crystals be grown in anatmosphere of an inert gas with a partial vapor pressure of materialfrom which the crystal is to be grown and that this vapor pressure beestablished in a tube which is initially opened and which is closed bycondensation of vapors of the charge upon a region in which condensationis sufficient to close off the reaction area, once a suflicient pressureof the charge substance has been obtained. Thus, for example, it hasbeen found that a similar apparatus may not be utilized is firstevacuated and sealed off and then the material isV heated to cause avapor pressure of the substance. One critical reason for this is thatwithout the balance of pressure supplied by the inert gas within tube 2,all materials known to be suitable for reaction tubes are so soft at theoperating temperature that the interior tubes collapse. In thisinvention, this is avoided by the use of the porous plug or similarclosure means. Additionally, the inert gas, by supplying a source ofconvection currents and a mediu-m of reasonable thermal conductivity,facilitates the essential features of removal of heat of fusion from thegrowing crystal and maintaining close control over the temperatureprofile within tube 2, which profile must change with respect to tube 2as the tube is moved relative to the profile.

It is further important, in the practice of the present invention, thatthe maximum temperature within the reaction vessel be only slightlyabove the temperature at Which the vapors of the charge condense to formthe single crystalline ingot. This is due to two factors. Greaterdifferentials in pressure may cause the establishment of two high avapor pressure within the chamber and cause a deformation of quartz tube2., as noted above. Additionally, single crystal nucleation seems to befavored by only a slight temperature gradient between the region inwhich vapors are formed and the region upon which the vapors condense toform the single crystal.

The following specific examples of the practice of the present inventionare set forth for purposes -of explanation only and are not to beconstrued in a limiting sense.

Example 1 The apparatus illustrated in FIGURES 1 and 2 was utilized. In.this apparatus a mullite tube havingian inside diameter of 3i land alength of 30 centimeters was utilized as tube 1. Tube 2 was of quartzand had an outside diameter of 5/s and a wall thickness of 1/16.

A charge of 50 grams of luminescent zinc sulfide activated withapproximately 0.01 weight percent each of copper and galliu'm wassintered by heating in a 1/2" diameter Crucible for approximately 1/2hour at ll00 C. in 1 atmosphere of hydrogen sulfide gas. The length ofthe sintered charge was l2 centimeters. This charge was inserted intotube 1, nearly abutting against the tapered portion of region 4. Alecentimeter long, 1/2 outside diameter quartz plug was inserted intothe interior of tube 2. The tube was sealed, as illustrated, andevacuated to a pressure of approximately l micron. The temperature ofthe tube was raised by energizing all coils uniformly to approximately700 C. while pumping continued for 1 hour. Argon gas was introduced intothe tubes at a pressure of 1 atmosphere while the temperature profilewas established by connecting to coil 7 a .20-volt voltage sourcecausing 6.8 amperes of current to exist in coil 7. `Coil 8 was connectedto a 60-volt supply causing 8.9 amperes to existv therein. Coil 9 wasconnected to a 55-volt voltage supply causing a current of 8.0 amps toexist therein and coil 10 was connected to a 20,-volt supply causing 6.8amperes of current to exist therein. After the temperature profile hadbeen established with the tip S of tube 2 at a temperature of 1470" C.,the assembly of tubes 1 and 2 was pushed at a rate of 0.8 millimeter perhour into the prole (tothe right in FIGURE 2)y for a period'of 50 hours.At the end of that time it was found that a single crystalline ingot ofzinc sulde having a length of 1.8l centimeters had been formed withintube 2. This single crystal was removed and wafers cut therefrom. Thesewafers were found to luminescence green under ultraviolet excitation of3650 A.U., under cathode ray excitation and when connected to anelectric eld of approximately ,volts per centimeter strength.

Example 2 The apparatus described in Example l was utilized. A cadmiumsulde charge was for'med by heating 50 grams of cadmium sulfide,activated -with 0.01% by weight of copper and gallium in a |vertical1/2 diameter crucible at a temperature of 1000 C. for 1/2 hour. Theresultant charge wa-s inserted into tube 2, almost abut ting againsttapered portion 4. The same plug utilized in Example l was insertedintotube 2 and tube 1 was closed after tube 2 had beeninserted therein.Both tubes were evacuated to a pressure of approximately 1 micron, atwhich time the temperature along the entire length of tube 2 was raiseduniformly to approximately 600 C. This temperature maintained whilepumping for a period of 10 hours. AfterY the foregoing period of time, atemperature profile was established within tube 2 by connecting coil 7to a 20,-volt supply to cause 6.8 amperes of current to exist therein.Coil 8 was connected to a 48-volt supply to cause 5.4 amperes to existtherein. Coil 9 was connected to 32-volt supply to cause 4.2 amperes toexist. therein. Coil 10 was connected to a 22-volt supply to cause 6.8amperes of current to exist therein. The temperature at the tip of tu-be2 was approximately 1230 C. After thermal equilibrium had beenestablished, tubes 1 and 2 were pushed into the stationary temperatureprofile (to the right in FIGURE 2) at a rate of 0.3 millimeter per hour.After 60 hours, the coils were disconnected and the tubes allowed tocool. A single crystal 1.5 centimeters in length was readily removedfrom the quartz tube. Wafer-s cut from this crystal exhibitedphotoconductivity characteristics and, when irradiated by 3650 A.U.excitation or exposed cathode ray excitation, emitted in the infraredportion of the electromagnetic spectrum.

While the invention has been set forth herein with respect to certainkembodiments and specific examples thereof many modifications and changeswill readily occur to those skilled in the art.. Accordingly, I intendby the appended claims, to cover all such modifications and changes asfall within the true spirit and scope of the invention. A l

What I claim as new and desire to secure by Letters Patent of the United-States is:

1. The method-of growing single crystals of a compound semiconductivematerial of the zinc-cadmium sulfo-selenid-e group comprising a cationof group IIb of the periodic, table and an anion of group VIa of theperiodic table, which method comprises:

(a) placing a charge of said material in a partially lclosed tube;

l, (b) establishing a preselected temperature profile along said tubesuch that the te-mperature at a iirst portion of said tube is suicientlyhigh to vaporize said material and the temperature at a second portionof said tube is sufficiently low as to cause condensation of saidvaporized material thereat;

(c) maintaining said profile until said vapors fill said 1- tube 'andseal said partially closed tube by condensation and a single crystal ofsaid compound begins to grow at said second portion of said tube; and

(d) moving said tube with respect to said temperature profile so thatthe interface between said growing c rystal and lvapors of said chargeis always at a temperature at which vapors of said charge just begin tocondense.

2. The method of growing large single crystals of a compoundsemiconductive material of the zinc-cadmium sulfo-selenide groupcomprising a cation of group IIb of the periodic table and an anion ofgroup Vla of the periodic table which sublimes appreciably beforemelting which method comprises:

(a) placing a charge of said compound within a iirst reaction tubehaving a tapered end;

(b) closing the remaining end of said first tube with closure means forallowing vapors of said charge to escape from said tube and for sealingsaid tube by accumulation of condensed vapors of said charge;

(c) inserting said iirst tube in a second reaction tube;

(d) establishing within both of said tubes an atmosphere of a gas whichis non-reactive with vapors of said charge;

(e) establishing along said irst tube a temperature profile such thatthe temperature thereof in a region encompassing at least a portion ofsaid charge is suiiiciently high to cause sublimation thereof, thetapered end thereof is substantially at a temperature at which saidvapors can condense, and said closure means is in a region of rapidlydecreasing temperature below the temperature at which vapors of saidcharge condense;

(f) maintaining said temperature proiile until a partial vapor pressureof said charge is established in said first tube, causing vapors thereofto escape from said first tube through said closure means and becomecondensed thereupon to thereby seal said irst tube; and

(g) further maintaining said profile to cause at least one singlecrystal of said charge material to nucleate at the pointed tip of saidfirst tube and grow thereupon while longitudinally moving said rst tubewith respect to -said temperature profile to keep the interface betweensaid growing crystal and vapors of said charge always at a temperatureat which supersaturation exists and vapors of said charge just begin tocondense.

3. The method of growing single crystals of a compound semiconductivematerial selected from the group consisting of Zinc sulde, cadmiumsulfide, zinc selenide, cadmium selenide, zinc-cadmium selenide,zinc-cadmium suliide, Zinc salio-selenide, cadmium sulfo-selenide, Zincoxide and cadmium oxide and which method comprises:

(a) placing a charge of said material in a partially closed tube;

(b) establishing a preselected temperature profile such that thetemperature at a -tirst portion of said tube is sufiiciently high tovaporize said material and the temperature .at a second portion of saidtube is sufliciently low as to cause condensation of said vaporizedmaterial thereat;

(c) maintaining said prole until said vapors iill said tube and sealsaid partially closed tube by condensation and at least one singlecrystal of said compound begins to grow at said second portion of saidtube; and

(d) moving said tube with respect 'to said temperature profile so thatthe interface between said growing crystal and vapors of said charge isalways at a temperature at which vapors of said charge just begin tocondense.

4. The method of growing large single crystals of a compoundsemiconductive material selected :from the group consisting of zincsulfide, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmiumselenide, zinccadmium sulfide, zinc sul'fo-selenide, cadmiumsulfo-selenide, zinc oxide and cadmium oxide and which method comprises:v .-V(a) placing a charge of saidmaterial within a -frst reaction tubehaving a tapered end;

(b) closing the remaining end of said first tube with closure means -forallowing vapors of said charge to escape from said tube and for sealingsaid tube by accumulation of condensed vapors of s-aid charge;

(c) inserting said first tube in a second reaction tube;

(d) establishing within both of said tubes an atmosphere of a gas whichis non-reactive with vapors o-f said char-ge;

(e) establishing along s-aid rst tube a temperature profile such thatthe temperature thereof in a region encompassing at least a portion ofsaid charge is sufficiently high to cause sublimation thereof, thetapered end thereof is substantially at a temperature at which saidvapors can condense, and said closure means is in a region of rapidlydecreasing temperature below the temperature at which vapors of saidcharge condense;

(f) maintaining said temperature proile until a partial vapor pressureof said charge is established in said first tube, causing vapors thereoft0 escape from s-aid first tube through said closure means and becomecondensed thereupon to thereby seal said first tube; and

(g) further maintaining said profile to cause at least one singlecrystal of said charge material to nucleate the pointed tip of saidfirst tube and grow thereupon While longitudinally moving said iirsttube with respect to said temperature proiile to keep the interfacebetween said growing crystal and vapors of said charge always at atemperature at which supersaturation exists and vapors of said chargejust begin to condense.

5. The method of growing large single crystals of a compoundsemiconductive material selected from the group consisting of zincsulfide, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmiumselenide, Zinccadmium sulfide, zinc -sulfo-selenide, cadmiumsulfo-selenide, zinc oxide and cadmium oxide and which method comprises:

(a) placing a sintered powder charge of said material in a firstreaction tube having a iirst, tapered end and a second end;

(b) closing said second end of said first tube with closure means forallowing vapors of said charge to escape from said tube and for sealingsaid tube by accumulation of condensed vapors of said charge;

(c) inserting said first tube in a second reaction tube;

(d) establishing within both of said tubes an atmosphere of a gas whichis non-reactive with vapors of said charge;

(e) establishing a preselected temperature profile cornprising:

(aa) a irst region and a second region wherein the temperature issuiciently low as to condense vapors of said material therein, and

(bb) a third region intermediate said first and second regions whereinthe temperature is sufciently high as to vaporize said material;

(f) position-ing said yfirst tube in said prole in such a manner` thatsaid rst, tapered end and atleast a portion of said charge lie in saidthird region and said second end lies in said rst region so that saidportion of said charge vaporizes and lthe vapors conyden-se in said rstregion to equalize the pressure within said rst and second tubes, sealsaid closure means and form a dense boule in said 'rst tube;

(g) moving said rst tube with respect to said profile so that said bouleenters said third temperature region causing evolution of vaporstherefrom and said first, tapered yend enters said second temperatureregion causing nucleation and growth' of a single crystal of saidmaterial thereat, while maintaining said second end in said rsttemperature region;

(h) continuing said movement so lthat vthe interface be- ReferencesCited by the Examiner UNITED STATES PATENTS 2,890,939 6/1959 Ravich t23-294 2,947,613 8/1960 Reynolds et al. 23-294 3,019,092 1/1962 Rosi eta1. 23-294 FOREIGN PATENTS 1,163,905 10/1958 France.

OTHER REFERENCES Lawson et al.: Preparation of Single Crystals,Butterworth Publishing Co., 1958, pp. 21-25, 89-91.

NORMAN YUDKOFF, Primary Examiner.

G. HINES, A. l. ADAMCIK, Assistant Examiners.

1. THE METHOD OF GROWING SINGLE CRYSTALS OF A COMPOUND SEMICONDUCTIVEMATERIAL OF THE ZINC-CADMIUM SULFO-SELENIDE GROUP COMPRISING A CATION OFGROUP IIB OF THE PERIODIC TABLE AND AN ANION OF GROUP VIA OF THEPERIODIC TABLE, WHICH METHOD COMPRISES: (A) PLACING A CHARGE OF SAIDMATERIAL IN A PARTIALLY CLOSED TUBE; (B) ESTABLISHING A PRESELECTEDTEMPERATURE PROFILE ALONG SAID TUBE SUCH THAT THE TEMPERATURE AT A FIRSTPORTION OF SAID TUBE IS SUFFICIENTLY HIGH TO VAPORIZE SAID MATERIAL ANDTHE TEMPERATURE AT A SECOND PORTION OF SAID TUBE IS SUFFICIENTLY LOW ASTO CAUSE CONDENSATION OF SAID VAPORIZED MATERIAL THEREAT;